Question

如何表达

> (exp(17.118708 + 4.491715 * -2)/-67.421587)^(-67.421587)

结果

[1] NaN

，而

> -50.61828^(-67.421587)

应该基本上有相同的结果，给我

[1] -1.238487e-115

这让我发疯，我花了几个小时寻找错误。＆＃34; -2＆＃34;，在这种情况下，是函数的参数。我真的无法想到解决方案。谢谢你的帮助！

编辑：

当我添加括号

时，我看到了

> (-50.61828)^(-67.421587)

它也会导致

[1] NaN

...但这并不能解决我的问题。

Answer 1

这是因为根据C99标准实施pow。

更不用说OP的例子：(-50.61828)^(-67.421587)，数学上合理的(-8)^(1/3) = -2在R中不起作用：

(-8)^(1/3)
# [1] NaN

引自?"^"：

 Users are sometimes surprised by the value returned, for example
 why ‘(-8)^(1/3)’ is ‘NaN’.  For double inputs, R makes use of IEC
 60559 arithmetic on all platforms, together with the C system
 function ‘pow’ for the ‘^’ operator.  The relevant standards
 define the result in many corner cases.  In particular, the result
 in the example above is mandated by the C99 standard.  On many
 Unix-alike systems the command ‘man pow’ gives details of the
 values in a large number of corner cases.

我在Ubuntu LINUX上，所以可以帮助获得man power打印在这里的相关部分：

   If x is a finite value less than 0, and y is  a  finite  noninteger,  a
   domain error occurs, and a NaN is returned.

Answer 2

据我所知，-50.61828^(-67.421587)正在评估-(50.61828^(-67.421587))。 (-50.61828)^(-67.421587)也会导致NaN。

R表达导致NaN没有明显的原因

2 个答案: